The boat clear astern won't tack at the windward mark — can she trap me like that?
Yes. A boat clear astern must keep clear and give you mark-room, but nothing forces her to tack. While you're both on port approaching the windward mark, she can hold her course and sail straight past — and you can't tack onto her because she's sitting right there. Rule 18 owes you mark-room, not a tack. (Rule 12 keeps her clear; she is never required to round.)
<iframe width="560" height="315" src="https://www.youtube.com/embed/S0n_iKqLD6o" title="Sailing Kiwi" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://sailing.kiwi.nz/windward-mark/mark-room-you-dont-have-to-tack/">Source: Sailing Kiwi</a></p>